Since a diffraction grating made of silicon carbide (SiC) is stable at relatively high temperature and has a good thermal conductivity, it is especially suited for synchrotron orbital radiation (hereinafter referred to as SOR) and short-wavelength, high energy laser light whose energy heats up the diffraction grating.
Direct etching of a diffraction grating pattern on an SiC substrate by dry etching such as ion beam etching is not easy, however, because the resist layer for masking the substrate surface is etched faster than the SiC substrate. Though generally the etching speed of a substrate is improved when such ions reactive to the substrate is used (Reactive Ion Etching), SiC is still too hard. Even when a gas reactive to SiC (CHF.sub.3, for example) is used, the etching speed of the SiC substrate is very slow and the resist layer is exhausted faster than SiC, whereby correct grating profile is difficult to obtain.
Thus a conventional SiC diffraction grating uses a surface coating of soft metal (such as gold) in which the grating is mechanically engraved by ruling engine or such, as shown in FIG. 5. In this case, though, when the diffraction grating is irradiated by very strong light (such as SOR or high energy laser light) for a long time, the diffraction grating may distort or the surface coating may separate from the substrate due to the difference in the coefficient of thermal expansion.
A novel method of direct etching of an SiC substrate has been proposed, in which the SiC substrate is etched with high-output SOR in a gas atmosphere (Proceedings of 1990 Spring Conference, p. 500: Society of Applied Physics). This method can be used, however, only in synchrotron facilities which are yet scarce for industrial application.